From time immemorial, lecturers have used pointers to highlight topics under discussion. With the advent of the laser in 1960, a pencil thin beam of light became available of sufficient intensity to highlight any presentation, in fact it was the very intense nature of the spot on the screen that presented a health hazard because such coherent beam could cause irreparable damage to the eye. To date, helium-neon lasers of one milliwatts or less have been used by lecturers to highlight their presentations. Unfortunately, lecturers tend to become engrossed in their presentations and the laser beam is sprayed all over the audience as well as the screen, causing considerable discomfort to any member of the audience whose eye happens to be directly in the said laser beam.
In the present invention the laser beam is only emitted when the laser beam generator is directed towards the screen because it is activated by the incoherent light transmitted by small transmitters placed around said screen. A problem exists however, when the lecture theatre has fluorescence tubes which operate at for example 100 cycles per second. This repetitive pulsing of the theatre lights, can interfere with the operation of the present invention if the intensity is high enough because the transmitters placed around the screen to activate the laser beam generator also emit a sequence of light pulses. To avoid interference, the incoherent light transmitters used to activate the laser beam generator is operated at relatively high pulsed light frequencies, for example 40,000 cycles per second and the pulse emission is timed such that they occur when the fluorescent tube emission is at a minimum. In this way the possibility of the laser beam generator being falsely triggered is minimised if not eliminated altogether under normal theatre operating conditions. Strong, continuous light sources also have minimal effects on the false triggering of the laser beam generator due to the high frequency pulsed operation of the incoherent light pulsers and detection systems used on the pointer of the present invention.
In addition to the direct line of operation between the incoherent light transmitters and the detector on the laser beam generator the invention can also operate by having the incoherent optical transmitter directing its output onto the screen and the detectors on the laser beam generator picking up the incoherent light scattered off the said screen. Our invention can also operate with the incoherent optical transmitters embedded in the head of the laser beam generator and the detectors either placed around the edge of the viewing screen for direct detection of the transmitted radiation, or in front of, and facing said screen for activation via light reflected off said screen. However, switching by reflected light other than off the said screen must be eliminated because it could give rise to false triggering of the laser beam generator.
The optimum operation of the present invention is achieved when the number of cables are minimised and the laser beam generator becomes a stand alone system with its own internal battery pack which can operate the laser tube for a minimum of one hour continuously or for a much longer time if the operation is intermittent. This means that only the optical transmitters on the screen are connected to the electrical mains, the connecting lead being well out of the way of either the lecturer or any member of the audience.